Priority is claimed on Japanese Patent Application No. 2002-376504, filed Dec. 26, 2003, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a polarizing electrode for an electric double layer capacitor and relates to an electric double layer capacitor using the polarizing electrode.
2. Description of Related Art
An electric double layer capacitor utilizes electrical energy stored in an electric double layer which is formed at the interface between a polarizing electrode and an electrolytic solution.
The electric double layer capacitor has a large Farad level capacity and excellent charge and discharge cycle characteristics, and it is therefore used in such applications as a backup power source for electrical equipment and a vehicle-mounted battery.
Referring to FIG. 6, for example, an electric double layer capacitor 1 has two polarizing electrodes incorporated therein, namely a first electrode 2 and a second electrode 3. The first electrode 2 and the second electrode 3 are separated from each other by a separator 4.
The first electrode 2 and a first current collector (hereinafter also referred to as a cap) 5 that is disposed outside the former constitutes one electrode unit 7 and functions as an anode. The second electrode 3 and a second current collector (hereinafter also referred to as a casing) 6 that is disposed outside the former constitute another electrode unit 8 that functions as cathode. Activated carbon that has microscopic pores is preferably used for the first electrode 2 and the second electrode 3 that constitute the electric double layer capacitor 1 (see Patent Document 1).
As shown in FIG. 7, the two polarizing electrodes 11 and 12 made of activated carbon that constitute the electric double layer capacitor are impregnated with an electrolytic solution 15 consisting of a solvent and an electrolyte. Electrolyte ions 16 and 17 are adsorbed so as to congregate in the pores 18 and 19 of the activated carbon that makes the two polarizing electrodes 11 and 12 through salvation with the electrolytic solution 15, so that the polarizing electrode 11 and the electrode unit 13 form an anode while the other polarizing electrode 12 and the electrode unit 14 form a cathode.
The activated carbon that makes the two electrodes can be regarded as providing a place for the solvent and the electrolyte ions to act electrochemically with each other thereon. Thus, the physical properties and microscopic structure of the activated carbon are among the factors that have great influence on the performance of the electric double layer capacitor.
As another example of the electric double layer capacitor described above, such a capacitor is known as an electrode unit formed in a sheet with a metallic body having electrical conductivity made into a foil (hereinafter referred to as an electrically conductive metal foil) pasted using an electrically conductive adhesive to form an integral member which is wound. For the electrically conductive metal foil, for example, a foil made of a metal such as aluminum (Al) is preferably used as it is after being etched on the surface thereof.
One of characteristics required of an electrode for a capacitor of high output power (about 250 W per cell) intended for use in automobile is a low internal resistance and sufficient capacity that enables it to draw a large current.
The capacity of a capacitor can be increased by increasing the capacity per unit weight of electrode (F/g). In the case in which there is a limitation to the volume of the capacitor module for the installation on automobiles or the like, capacity per unit volume of electrode (F/cc) must be increased instead of capacity per unit weight of electrode (F/g). Increasing the capacity per unit volume of electrode (F/cc) means increasing the molding density of electrode.
For increasing the molding density of an electrode, such methods are known as increasing the density of activated carbon without decreasing the capacity per unit weight, or molding the electrode in a close-packed structure.
As the former method, that is, to increase the density of activated carbon, Patent Document 1 discloses a method in which an easy-to-graphitize material is used to make the activated carbon, so that a carbon material obtained by carbonizing the former at a temperature of 1000° C. or less in an inert atmosphere is activated with a hydroxide of an alkali metal, thereby producing activated carbon.
However, since it is difficult to control the activation process in a manufacturing process that produces activation using a chemical, and requires a process of washing off the chemical to such a level that it does not affect the operation of the capacitor after the activation, many problems remain to be solved from the viewpoint of manufacturing cost, before the process can be employed for mass production.
For stable production of activated carbon, it is known to activate carbon with a gas such as water vapor, instead of the chemical. In this case, carbon made by carbonizing a hard-to-graphitize material at a temperature around 1000° C. in an inert atmosphere is used. In the case of this method, there has been a problem in that since a hard-to-graphitize material which is relatively easy to activate is used, formation of microscopic pores in the activated carbon proceeds excessively and, as a result, density of the activated carbon tends to decrease.
As methods of the latter category, that is, to form the electrode having a close-packed structure, there are such methods as the density of an electrode sheet is increased by controlling the load of rolling when forming the electrode sheet (see Patent Document 2), and a method of controlling the particle size of the activated carbon that is the main component (see Patent Document 3).
However, an electrode formed to have a high density by any of the methods described above has problems such as cracking, rupturing or other significant molding defects occurring in the molded sheet or problems such as a decrease in the infiltration rate of the electrolytic solution or insufficient impregnation occur in the process of impregnating with the electrolytic solution during assembly of the capacitor.
There has also been a problem in that it is difficult to determine whether the activated carbon is good as a stock feed in advance, since the moldability and the electrode density can be evaluated only after the material is formed into a sheet.
[Patent Document 1] Japanese Patent Application, First Publication No. Hei 9-320906
[Patent Document 2] Japanese Patent Application, First Publication No. 2000-277391
[Patent Document 3] Japanese Patent Application, First Publication No. 2001-52972